It is known to implant medical devices such as stents, stent-grafts, vena cava filters, and so forth, into body lumens, such as arteries to reinforce, support, repair or otherwise enhance the blood flow through the lumen.
Stents are commonly used where an artery is blocked, or otherwise damaged. The stent, once in place, reinforces that portion of the artery allowing normal blood flow to occur through the artery. One type of stent that is popularly used for such purposes is a radial expandable stent. This is a tubular or cylindrical stent which can be radially expanded from a first smaller diameter to a second larger diameter. These stents are either self-expanding, or are pressure-expandable.
The stents are inserted into an artery through the use of a stent delivery device and are fed internally through the arterial pathways of the patient until the unexpanded stent is located where desired. The catheter may either be fitted with a balloon in the case of a pressure expandable stent, or with stent retaining sleeves in the case of a self-expanding device. These expandable stents have properties such that they remain expanded after the catheter has been removed.
One problem encountered in the use of radially expandable surgical stents is the difficulty in precisely determining the position of the stent both before, during and after it is implanted and expanded. Stents are typically formed of metals or metal alloys including stainless steel, shape memory alloys such as nickel-titanium alloys, or some other such alloy which in and of themselves are not readily visible using fluoroscopic imaging techniques. It is of particular importance to be able to clearly and accurately visualize the stents during the time that they are being deployed and also to be able to visualize the stents after they have been deployed, at periodic time intervals.
Attempts have been made to make such stents more radiopaque through the use of various metals such as platinum, tantalum, gold, and so forth. One such approach has been to fabricate the stent itself from such materials, such as tantalum, but the stents fabricated of radiopaque materials have been found to illuminate too brightly, resulting in haloing or obscuring of fine details such as blood vessels and other bodily structures, thus making it difficult to visualize the vessel or lesion which is being repaired.
Another approach has been to coat the stent with such materials. Several problems may be encountered using such coating methods including restriction of the expansion capabilities of an expandable stent by adding rigidity to the stent in areas designated for stent deformation, the coating can flake off of the stent if it is not securely anchored to the stent surface, and completely coating a stent can also result in haloing, a phenomenon resulting from the radiopaque material illuminating too brightly so as to obscure proper visibility of the blood vessel lesion, thereby impairing the ability to repair the lesion. Also, coatings which are too thick will significantly enlarge the thicknesses of the stent making this technique less effective on stents sized for implantation in smaller body lumens, and if the coating is too thin, insufficient radiopacity will result.
A partial or selective coating can reduce the occurrence of the above mentioned problems, but can also result in decreased efficiency in the manufacturing process. For instance, some methods of partial coating involve the added step of masking the stent in certain areas. The masking procedure is difficult to accurately perform, especially on inner surfaces of stents with small diameters.
Radiopaque markers can also be attached by mechanical or adhesive means, for instance. These methods too can have various limitations. Upon attachment to a stent, they may define a profile that is readily discernible from that of the stent, thereby comprising projections which may undesirably alter the contemplated profile of the stent. That is, they may protrude from the walls of the stent and depending upon their location upon the stent, may either project into the blood flow or into the walls of the blood vessel. Such markers can also be tedious to attach to the stent resulting in decreased efficiency in production, and they can also be difficult to attach in a precise location.
U.S. Pat. No. 5,741,327 describes the mechanical attachment of marker elements to the ends of a stent with the axial center of the marker elements being coextensive with the central axis of the stent. The marker element is configured to be radially expandable in a manner similar to the radial expansion of the stent itself. However, the marker elements are attached to the ends of the stent in positions beyond the ends of the stent. The marker elements can either be circumferentially continuous, completely circumscribing the central axis of the stent, or discontinuous with a series of disconnected marker elements secured to the ends of the stent at separate positions thereon.
There continues to be a need in the art for a new and improved radiopaque markers for use on radially expandable stents which can be utilized on stents of all different sizes, do not interfere with the expansion characteristics of the stent and provides a clear image on a fluoroscope or other medical imaging device. Furthermore, there remains a need in the art for a simple method of fabrication for such radiopaque stents that allows the fabrication using known and efficient manufacturing techniques.